It is desired that a photosensitive resin plate for flexographic printing can be developed with water rather than organic solvent, because of easy treatment, worker's health, safety and environmental pollution. A printing plate having a relief layer obtained from the photosensitive resin plate is contacted with ink, particularly aqueous ink or alcoholic ink. The relief layer accordingly has to have suitable chemical resistance against the solvent of the ink. It is also required that the printing plate keeps its physical or chemical properties during the printing operation.
Japanese Kokoku Publication 53-35481 discloses a liquid type photosensitive resin composition which contains a liquid unsaturated polyester as a main component. The composition is liquid and therefore it is difficult to obtain precise film thickness on a substrate. The composition necessitates the use of an air knife for developing.
U.S. Pat. No. 4,369,246 discloses a solid type photosensitive resin composition which, however, is developable with organic solvent and therefore does not meet the trend toward water developing.
Water developable photosensitive resin compositions for flexographic printing are proposed in U.S. Pat. Nos. 2,893,868, 4,275,142 and Japanese Kokai Publication 61-22339. However, the compositions disclosed in the two U.S. Patents have poor water resistance of the resulting printing plate and also do not have sufficient water developability. The composition of the Japanese Publication has poor mechanical properties after developing.
U.S. Pat. No. 4,177,074 and Japanese Kokoku Publication 60-45416 suggest a combination of a high molecular weight polymer prepared from a high molecular weight conjugated diolefin hydrocarbon and a low molecular weight polymer prepared from a low molecular weight conjugated diolefin hydrocarbon, which improves developability. This composition, however, does not improve mechanical properties.
The present inventors have proposed in Japanese Kokai Publication 1-300246 (corresponding U.S. Pat. No. 5,073,477) a photosensitive resin composition for flexographic printing which comprises (i) crosslinked polymer particles and (ii) a linear polymer with at least 5,000 molecular weight, which contains at least 30 mol % of conjugated diene units in one molecule. The composition has good water developability and when developed has good mechanical properties.